In the past various attempts have been made to design hypodermic syringes with retractable needles. Typical examples of such devices are shown in U.S. Pat. Nos. 4,838,863; 5,019,044; 5,064,419; 4,950,241; and 4,978,343.
Some of these patents show the needle retracted into a hollow piston or barrel of a syringe either manually or by a spring which is biased to move the needle into a stored position either within a hollow piston or at least within the barrel of a hypodermic syringe. Such devices are only as effective and reliable as the design of the mechanisms used to retract the needle and some mechanisms may either fail to retract the needle completely or may fail to retain the needle in a retracted position.
The present invention is a further development of the concept shown in prior U.S. Pat. No. 5,180,370 issued to E R Gillespie which uses a hollow plunger in a medical syringe as a needle storage compartment when the needle has been retracted inside the syringe after an injection has been given. One primary advantage of the hollow plunger is that the needle can be in the stored or retracted position inside the plunger when the plunger is pressed into the barrel. In other patents which do not show a hollow plunger, the plunger must either be left protruding from the rear end of the barrel after needle retraction or in some instances the protruding portion of the plunger is broken off at the rear end of the barrel. Either option is not as desirable as having substantially all of the plunger contained inside the barrel after the needle has been retracted.
Both the prior Gillespie patent (U.S. Pat. No. 5,180,370) and the present invention provide a positive means of hydraulically sealing off the interior of the hollow plunger from the fluid chamber of the syringe.
The prior Gillespie patent mentioned above, uses a rupturable end cover member over the front end of the plunger to seal off the interior of the hollow plunger from the fluid chamber and an annular cutting surface to rupture or separate the cover member from the plunger while releasing a needle retaining member to permit the retaining member with a needle to be transmitted into the hollow interior of the plunger.
The present invention uses a positive and realiable end closure cover for sealing off the interior of the plunger from the fluid chamber and an improved angled cutting edge for shearing off and cutting loose the end closure cover or wall and an improved angled cutting edge on the front end of the plunger for shearing or cutting loose the needle retaining assembly from the barrel so that it is propelled by a bias means into the hollow interior of the plunger and retained therein.
Many additional patents have been issued on retractable needles since the prior Gillespie patent. Typical examples of such patents are U.S. Pat. No. 5,188,599 (Botich et al.); U.S. Pat. No. 5,190,526 (Murray et al); and three U.S. Pat. Nos., 5,267,961; 5,389,076; and 5,423,758 (all of which are issued to T R Shaw). The present invention differs from these patents in both the manner in which the end of the plunger is closed and in the manner in which the needle is retained in the front end of the barrel and released therefrom upon depression of the plunger.